The present invention relates to an electric pump in which a rotary shaft rotated by an electric motor is supported in a housing. A rotor, which rotates along with rotation of the rotary shaft, is accommodated in a pump chamber.
For example, Japanese Laid-Open Patent Publication No. 8-242565 discloses a centrifugal pump that rotates a rotor to transfer fluid. The centrifugal pump disclosed in Japanese Laid-Open Patent Publication No. 8-242565 has a permanent magnet synchronous motor to rotate a rotary shaft. As the shaft is rotated, an impeller (rotor) accommodated in a casing is rotated. By means of centrifugal force produced by the rotation of the impeller, fluid (liquid) is transferred. The permanent magnet synchronous motor of the centrifugal pump has a permanent magnet rotor inside of a stator. The permanent magnet rotor is accommodated in a cup-shaped element and has an integrally formed power transmitting tooth. The shaft has a ridge that rotates integrally with the shaft. A lid, to which the ridge is attached, seals the cup-shaped element such that the lid rotates integrally with the cup-shaped element. Further, a shock-absorbing element made of elastomer is accommodated in the cup-shaped element. The shock-absorbing element is arranged between the power transmission tooth and the ridge with respect to the moving direction of the tooth.
In the centrifugal pump disclosed in Japanese Laid-Open Patent Publication No. 8-242565, when the power transmission tooth is rotated as the permanent magnet rotor in the permanent magnet synchronous motor rotates, the tooth freely rotates in a predetermined rotation angle. Thereafter, the power transmission tooth contacts the shock-absorbing element, and the shock-absorbing element then contacts the ridge. This rotates the lid and the cup-shaped element. Since the lid is coupled to the shaft, the shaft is rotated. Further, as the shaft rotates, the impeller rotates to achieve the pumping action. During the operation of the centrifugal pump, the shock-absorbing element applies frictional force to the wall of the cup-shaped element. The frictional force reduces the shock produced when the power transmission tooth contacts the ridge. Such reduction of shock results in a lowered level of knocking noise.
Fuel-cell vehicles, which have a fuel-cell system, are now attracting attention. A fuel cell in such a fuel-cell system causes hydrogen and oxygen to react with each other to generate electricity. The vehicle drives by means of electricity generated by the fuel cell. A fuel-cell system includes a hydrogen circuit for re-supplying unreacted hydrogen gas (“hydrogen off-gas”), which has not been used in the fuel cell, to the fuel cell. The hydrogen circuit has a pump for transferring hydrogen off-gas. The pump is driven by force of an electric motor.
As the fuel cell generates electricity, water is produced. The water is discharged from the fuel cell together with the hydrogen off-gas, and the water and the hydrogen off-gas are sent to the pump chamber in the pump. When the fuel-cell system operates in a cool environment, the water drawn into the pump chamber is condensed between a rotor (for example, pump rotor) accommodated in the pump chamber and the inner wall of the pump chamber or on an outer surface of the rotor. Further, if the fuel-cell system is stopped after being operated in a cool environment below freezing, the condensed water will be frozen, which may adhere the rotor with the inner wall of the pump chamber. If the rotor is adhered with the pump chamber inner wall, the rotor needs to be detached from the pump chamber inner wall when restarting the fuel-cell system. Therefore, in the case where the rotor and the pump chamber inner wall are adhered to each other, a great torque needs to be generated when restarting the system, so as to detach the rotor from the inner wall. Accordingly, a large sized electric motor capable of generating the great torque is needed. The size of the pump therefore needs to be increased.
In the centrifugal pump disclosed in Japanese Laid-Open Patent Publication No. 8-242565, friction is positively produced between the shock-absorbing element and the cup-shaped element, so that the shock between the power transmission tooth and the ridge is reduced. Therefore, when activating the centrifugal pump, the torque generated at the shaft is small, and it is impossible to detach the rotor (an impeller in Japanese Laid-Open Patent Publication No. 8-24256) from the inner wall of the rotor chamber by such a small torque in the case of adhesion due to frozen water as described above.